WFC3 Instrument Handbook for Cycle 24

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7.2	Specifying an IR Observation

In the HST proposal system, the parameters for an IR observation available to General Observers are:

1.	Configuration: always WFC3/IR.

2.	Mode: always MULTIACCUM.

3.	Aperture: must be specified; see Sections 7.4.4 and 7.4.5 for the dimensions, location, and reference point (target placement) of the full array and subarray apertures.

4.	Spectral Element: must be specified for each exposure; see Section 7.5.

5.	Optional Parameters: in the IR channel SAMPSEQ specifies a predefined sequence of times at which the detector is read out, and NSAMP specifies the total number of readouts (and thus the total exposure time); see Section 7.7. SAMPSEQ and NSAMP are required for IR observations.

6.

Special Requirements: see the Phase II Proposal Instructions for details of Special Requirements related to the timing of visits and for dithering and mosaicking. Also available is the exposure-level Special Requirement POS TARG for offsetting the target from the default reference point of the aperture (see Section 7.4.3 for a discussion of the IR channel coordinate system).

7.

Number of Iterations and Time per Exposure: in cases where two or more identical exposures are desired, the Number of Iterations may be set to 2 or more. In the IR channel, the Time per Exposure is fixed by the combination of SAMPSEQ and NSAMP (see item 5 above) and thus is not specified separately. Through various combinations it is possible to select times per exposure ranging from 2.93 to 2756 s (for readouts of the full detector array; considerably shorter exposures are possible using subarrays).